Gas turbine engines, such as those which power aircraft and industrial equipment, employ a compressor to compress air that is drawn into the engine and a turbine to capture energy associated with the combustion of a fuel-air mixture. A bearing compartment, and associated bearings, provides support for one or more structures of the engine, such as for example rotating shafts of the engine.
Referring to FIG. 2A, a prior art system 200 for scavenging oil is shown. Oil from one or more input sources is provided to a bearing compartment 204 to clean, cool, and lubricate the bearing(s) contained therein. Seals 210a and 210b are coupled to the bearing compartment 204. At least during steady state/idle conditions associated with the operation of the engine, the seals 210a and 210b are provided buffer air (typically sourced from the compressor) that is at a higher pressure than the pressure of the bearing compartment 204. This high pressure air, in conjunction with the seals 210a and 210b themselves, helps to retain oil in the bearing compartment 204. In other words, the high pressure buffer air, in conjunction with the seals 210a and 210b, keeps the oil from leaking out of the bearing compartment 204. Secondary seals 216a and 216b may be used to further encourage the buffer air to enter the bearing compartment 204 via the seals 210a and 210b. 
During the steady state/idle conditions described above, a valve 222 disposed in a scavenge line 228 may be substantially closed (the valve 222 may remain partially open so as to continue to provide a threshold amount of oil to a fluid reservoir/sink 234 coupled to the scavenge line 228, where the sink 234 may include or be coupled to a gearbox or other components/devices, and where the sink 234 may be coupled to the source of the oil that is provided to the bearing compartment 204 in a closed-loop manner). The substantial closure of the valve 222 may provide a restriction to the flow of oil and air from the bearing compartment 204 to the sink 234 by way of the scavenge line 228. This restriction in the flow pressurizes the bearing compartment 204 and reduces the differential pressure across the seals 210a and 210b, thereby extending the operational lifetime and reliability of the seals 210a and 210b, as well as reducing heat generation.
Upon a snap/transient condition (e.g., a rapid deceleration of the engine), the pressure associated with the buffer air may drop appreciably. To counter this, the valve 222 may be fully opened in an effort to provide minimal restriction to the flow of oil and air out of the bearing compartment 204. However, the rate at which the source buffer air pressure drops may be significantly greater than the rate at which the bearing compartment 204 is able to depressurize via the valve 222/scavenge line 228, such that a negative (reversed) differential pressure may exist across the seals 210a and 210b and the risk of oil leaking out of the bearing compartment 204 is elevated (e.g., is greater than a threshold).
FIG. 2B illustrates another prior art system 250 for scavenging oil. The system 250 includes many of the same components/devices described above with respect to the system 200 of FIG. 2A. As shown in FIG. 2B, the system 250 includes a scavenge pump 278 in lieu of relying on compartment pressure to scavenge the compartment. If the pump 278 capacity/flow rate is small (e.g., less than a threshold), the system 250 may provide for a desirably small differential pressure across the seals 210a and 210b during steady state/idle conditions; however, upon the occurrence of a snap/transient condition as described above the pump 278 may be ineffective in depressurizing the bearing compartment 204, which may result in leaking oil. On the other hand, if the pump 278 capacity/flow rate is large (e.g., greater than a threshold), the system 250 may be able to quickly depressurize the bearing compartment 204 upon the occurrence of a snap/transient condition. However, during steady state/idle conditions there may be an unnecessarily/undesirably large differential pressure that may exist across the seals 210a and 210b. 
Accordingly, what is needed is an ability to minimize/reduce the differential pressure across the seals of a bearing compartment during a steady state/idle condition on the one hand, yet an ability to quickly depressurize the bearing compartment during a snap/transient condition on the other hand.